1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery.
2. Description of the Related Art
In recent years, a nonaqueous electrolyte secondary battery such as a lithium-ion secondary battery or a polymer lithium secondary battery is widely used in an electronic appliance such as a portable telephone. These secondary batteries of this type have a lightweight, a high energy density, and provide a high operating voltage near 4V compared with a nickel-cadmium battery or a nickel-metal hydride secondary battery. The excellent performance of the secondary battery of the type noted above attracts keen attentions in this technical field.
In recent years, it is studied to use the particular nonaqueous electrolyte secondary battery as a power source for an electric vehicle, an electric tool, uninterruptible power supply or a cordless cleaner. In such a use, required is high power performance, compared with a nonaqueous electrolyte secondary battery used in an electronic appliance such as a portable telephone.
In order to increase the high rate discharge capacity of the battery for obtaining a high power battery, it is necessary to decrease the internal resistance of the battery as much as possible. For decreasing the internal resistance of the battery, it is important to lower the resistance of the electrode and the component parts to increase the current collecting efficiency. A measure for achieving the object is the development of a cylindrical lithium-ion secondary battery or a prismatic lithium-ion secondary battery comprising a plurality of current collecting leads connected to an electrode, as proposed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 9-92335. The particular construction makes it possible to improve the current collecting efficiency, to lower the internal resistance of the cell, and to obtain a battery excellent in the high rate discharge characteristics. Also, in a high rate discharge type lithium secondary battery of this type, it is possible to charge the secondary battery with a large current so as make it possible to achieve a rapid charging.
In a nonaqueous electrolyte secondary battery, it is strongly required in general to ensure the safety in the event of an abnormal increase in the internal pressure of the cell caused by the inappropriate handling such as the situation that the secondary battery is left to stand under a high temperature atmosphere for a long time, an overcharge, or an external short-circuiting, not to mention the safety of the secondary battery during the normal use. In order to ensure the safety, the nonaqueous electrolyte secondary battery is provided with a safety vent that is operated by the internal pressure of the cell battery, thereby preventing the explosion of the battery. In the nonaqueous electrolyte secondary battery of this type, the safety vent is opened when the internal pressure of the cell is increased to exceed a designed operation pressure to release the gas filling the inner space of the battery container to the outside through the opened safety vent, thereby preventing the explosion of the battery.
However, in the high rate discharge type nonaqueous electrolyte secondary battery, the operating pressure of the safety vent is nonuniform, giving rise to the inconvenience that it is possible for the safety vent not to be operated even in the case where the internal pressure of the cell is increased to reach a designed operation pressure by the occurrence of an abnormality such as an overcharge.